1. Field of the Invention
The present invention relates to an optical information recording/reproduction apparatus for recording and/or reproducing information on and/or from an optical information recording medium, on which information is stored in an optically detectable format.
2. Related Background Art
A conventional reproduction apparatus for an optical information recording medium normally has an arrangement, as shown in FIG. 1A. More specifically, light emitted from a semiconductor laser 1 is collimated by a collimator lens 2, and is then guided to an objective lens 4 via a beam splitter 3, thus forming a small spot on a disk 5. At the same time, the small spot is stably scanned along an information track by auto-focusing and auto-tracking control means (neither are shown). On the optical information recording medium used in this case, an information pit array having a predetermined thickness is formed. Normally, the thickness of each information pit is set, so that the phase difference between light beams reflected by an information pit formation area and a non-formation area becomes almost .pi.. For this reason, when the small spot reaches a position above an information pit, the reflected light quantity considerably decreases due to diffraction. Therefore, an information detection system obtains a change in total reflected light quantity as a reproduction signal. More specifically, as shown in FIG. 1A, reflected light from an information storage medium 5 is received by the objective lens 4, and is guided by the beam splitter 3 to a detection system. Then, the light is received by a photodetector 16 via a condenser lens 15, and the photodetector 16 converts the received light into an electrical signal. FIGS. 2A and 2B show a reproduction signal obtained in this manner. When a small spot 14 moves on an information pit 5a, as shown in FIG. 2A, the reflected light quantity changes, as shown in FIG. 2B, and the light quantity sharply changes near the edge of the information pit 5a. When this signal is binarized with a threshold level I.sub.t, an information signal can be obtained.
When the photodetector 16 has two light-receiving surfaces 16a and 16b, which are split by a split line passing the optical axis in a direction perpendicular to a track direction, as shown in FIG. 1B, a device for detecting an edge so that peak outputs are output at the leading and trailing edges of an information pit by calculating a difference between detection signals of the two light-receiving surfaces by a subtracter 17, is known (e.g., disclosed in Japanese Patent Application No. 58-83082).
However, when information is written on a storage medium at a high density so as to attain the object of increasing the storage capacity, a stable reproduction signal cannot be obtained since a change in reflected light quantity is very small with respect to an information pit sufficiently smaller than the size of the small spot 14, as shown in FIGS. 3A and 3B. Also, a signal obtained by calculating a difference between signals from the above-mentioned two-divided photodetector suffers from a shift in peak position of a reproduction signal between a case wherein the reproduction spot reaches the leading edge and a case wherein the spot reaches the trailing edge. Thus, the edges cannot be precisely reproduced.